Xxencoding is obsolete but still included for completeness. Further
information on the encoding can be found at
http://en.wikipedia.org/wiki/Xxencode. It should be noted that this
implementation performs no padding, due to the splitting up between encoding
and chopping.

Chop up a string in parts. Each string in the resulting list is prepended
with the length according to the xxencode "specificiation".

Notes:

The length of the strings in the result will be (n -1) div 4 * 4 +
1. The -1 comes from the need to prepend the length (which explains
the final +1). Keeping it to a multiple of 4 means that strings
returned from encode can be chopped without requiring any changes.